Physiological time-series signals measured from the human body or other organism often constitute of two summed signals, where one is of oscillatory transient type, an oscillatory signal, and the other one a modulating signal. The modulating signal is continuous periodic signal that has lower frequency than the oscillatory signal. The oscillatory signal occurs at certain time intervals due to a triggering event. The trigger can be internal, like the heart sinus node activation starting a heart contraction, or external, like a flash of light triggering an evoked potential in the brain. Oftentimes the measured signals caused by these oscillatory events change shape due to some external factor. For instance, the shape of the electrocardiographic signals recorded for each heart contraction change shape with respiration. The oscillations and the modulating signal can also be summed together as is the case in the thoracic cardiac and respiratory impedance signals.
In impedance pneumography, the transthoracic electrical impedance varies over time due to the cardiac function and the respiration. Cardiogenic impedance signal Zc, that is cardiogenic part of impedance signal Z, originates from the movement of blood volumes in the thorax, and the respiratory impedance signal Zr, that is respiratory part of impedance signal Z, is directly proportional to the lung volume. These measurable signals can be exploited to analyze cardiac function, as in impedance cardiography (ICG), or lung function, as in impedance pneumography (IP). For reliable analysis of the pulmonary variables of interest should the cardiogenic impedance signal Zc, an additive noise signal, to be suppressed, because presence of the cardiogenic oscillations (CGO) hinders the accurate segmentation of the impedance signal into respiratory cycles and finding of the points of interest, like time of peak expiratory flow. Preserving the harmonic components of the respiration signal is important in the emerging IP applications, like ambulatory long-term lung function assessment, where tidal breathing parameters more complex than respiration rate or tidal volume may be extracted from the impedance signal Z.
The frequency spectra of the cardiogenic impedance signal Zc and the respiratory impedance signal Zr have their corresponding main power components at the frequencies of heart rate (HR) and respiration rate (RR), respectively. The main cardiac component is typically at a frequency at least two times higher than that of the respiration. However, the harmonic frequencies of the cardiogenic impedance signal Zc contain power that reach the HR frequency causing the power spectrum of the two signals to overlap.
Thus, if CGO are removed the with a normal linear low pass filter with cut-off frequency slightly below the HR, some information of the respiratory impedance signal may also be removed. This problem may be pronounced in subjects with high RR to HR ratio.
European patent EP434856B1, “Method of deriving a respiration signal and/or a cardiac artefact signal from a physiological signal,” discloses a method of deriving a respiration signal and/or a cardiac artefact signal from a physiological signal having at least a respiration signal component and a cardiac artefact signal component, in particular from an impedance pneumography signal. However, this method does not recognize the interaction between the cardiac oscillation signal and the lung volume. On the contrary, it is stated that the cardiac artifact signal has a waveform which in terms of time remains substantially the same from one heartbeat to another heartbeat.
There is, therefore, a need for a solution that attenuates the cardiogenic oscillations in impedance pneumography signal by taking into account the modulating effect that the changing lung volume has on the cardiogenic oscillation waveform.